Rooster Cogburn is a great Musketeer companion at lower levels, though in higher level worlds he begins to fall behind. When he is recruited, give him Burst Fire 2; this will allow him to land multiple extra hits on opposing enemies, particularly Buccaneers, Privateers, and Witchdoctors. With his True Grit, he will be able to strike back against other Musketeer enemies and possibly attack with several Burst Fires due to that single hit.

If you are interested in lower level PvP, especially in the 30-50 range, Rooster Cogburn is a great choice as an additional Musketeer on your team. With companions such as Hawkules and Zeena not being available at those levels, as well as other existing companions who have not yet promoted to their final tiers, Rooster Cogburn is one of the best choices at those levels.